Dixon & Wigham — Radiations from Radium Bromide. 183 



Illustrates this point. In the centre of the sterile patch may be seen 

 a faint cross. It is the mechanical trace of an unsuccessful inocu- 

 lation by means of a platinum needle. The photograph was taken 

 two days after inoculation, during which time the culture was in- 

 cubated at 37 c 0. The inoculation, however, failed to develop. 

 This, and similar experiments, showed that the medium had 

 become unsuited to the development of bacteria. 



We have made some observations to determine whether this 

 unsuitability is brought about by the action of the radiations, or 

 by the diffusion of substances from or into the culture outside. 



There are evidently two ways in which this question may be 

 approached. (1) "Will a plate devoid of bacteria become unsuit- 

 able for bacterial development where exposed to the radiations ? 

 (2) Will diffusion from or into a culture surrounding a sterile patch 

 (not sterilized by the radiations) render it unsuited for the growth 

 of bacteria ? 



Our results obtained while endeavouring to solve the question 

 in the former of these two ways indicate that the unsuitability of 

 the medium in the sterile patch is not directly due to the radia- 

 tions. In these experiments plates were exposed to the radiations 

 for from three to four days, and then inoculated by smearing their 

 surface with an emulsion of bacteria. In each case the culture 

 developed all over the plate, and showed no sterile patch compar- 

 able to that produced when the culture itself is exposed. But 

 these experiments were rendered somewhat inconclusive from the 

 fact that some unexposed medium was introduced with the 

 inoculation, and consequently must be repeated without this 

 possible source of error. 



In order to test if diffusion from or into the surrounding 

 culture would render the sterile area unfit for growth, we 

 carried out several experiments in the following manner : — 

 Emulsions of bacteria were poured out on glass plates. When the 

 agar had set, circular patches, 15 mm. in diameter, were removed 

 by pressing the rim of a sterilized test-tube into the agar. The 

 patch thus removed was replaced by melted agar containing no 

 bacteria, and so an uninoculated patch was obtained, surrounded by 

 a culture containing developing bacteria. After incubation for 

 three or four days the sterile patch was inoculated. Development 

 of the inoculation did not take place when time was given for 



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